The latest fad in self-powered wrist wear is the kinetic watch, a device that converts the momentum of a swinging arm into milliwatts. But researchers have unveiled a new accessory for your knees that puts the trendy timepiece to shame. Generating more than 1000 times more energy, the "Biomechanical Energy Harvester" may provide a green way to power the portable devices of the future.

Every time you take a step, you use two different groups of powerful muscles connected to the knee. The first group pushes to kick the lower leg out. Just before full extension, the second group pulls to put the brakes on. But for Max Donelan, director of the Simon Fraser University Locomotion Laboratory in Burnaby, Canada, and his colleagues, this braking process is just useful energy going to waste. His team has created a modified knee brace with a drive train that converts the mechanical energy into electricity. "A similar principle is used in hybrid cars to make electricity when you press the brakes; it's called generative braking," says Donelan.

Six volunteers wore the braces while they walked on treadmills. Embedded sensors detected the angle and velocity of their legs, switching the device on only during the braking phase of each swing. As the team reports tomorrow in Science[2], the braces produced 5 watts of power--enough to run 10 cell phones. And although it took a bit more effort to swing the added weight of the brace--the prototype weighs 1.6 kg--the walkers didn't have to work harder when the power-harvesting mechanism was turned on. The amount of oxygen they consumed--a measure of metabolism and effort--didn't increase. "Our generator actually helps your muscles out," says Donelan, "by decelerating your limbs for you."

If the researchers can lighten the load of the device, the first users will likely be people whose lives depend on reliable, portable power: patients with insulin pumps, for example. Douglas Weber, a team member and mechanical engineer at the University of Pittsburgh School of Medicine in Pennsylvania, believes that it will also be incorporated into the design of cutting-edge neuroprostheses--artificial limbs directly controlled by brainwaves and deep-brain stimulators for Parkinson's disease patients. Eventually, the device might prove useful for anyone off the main power grid: soldiers, relief workers, hikers, even normal folks with cell phones and personal digital assistants.

According to Lawrence Rome, a biologist at the University of Pennsylvania who designed a biomechanical backpack years ago (ScienceNOW[3], 8 September 2005), this is the most sophisticated attempt to harness biomechanical energy to date. "Other people have thought about trying to get energy out of joints," says Rome, "but only Donelan and his team saw the opportunity to use braking motion." He wonders what other powerful joints we could tap for energy--shoulders, maybe, or even elbows.